The Nio (Benevolent Kings) are a pair of protectors who stand guard outside the temple gate at most Japanese Buddhist temples, one on either side of the entrance. In Japan, the gate itself is often called the Nio-mon (literally Nio Gate). Their fierce and threatening appearance wards off evil spirits and keeps the temple ground free of demons and thieves. In some accounts, the Nio were said to have followed and protected the historical Buddha when he traveled throughout India. They have since been adopted by the Japanese into the Japanese Buddhist pantheon.

Each is named after a particular cosmic sound. The open-mouthed figure is called "Agyo," who is uttering the sound "ah," meaning birth. His closed-mouth partner is called "Ungyo," who sounds "un" or "om," meaning death. Other explanations for the open/closed mouth include:
(1) mouth open to scare off demons, closed to shelter/keep in the good spirits;
(2) "Ah" is the first letter in the Sanskrit alphabet and "Un" is the last (same in Japanese syllabary too), so the combination symbolically represents all possible outcomes (from alpha to omega) in the cosmic dance of existence.

The most famous rendering of A-Un are probably the huge statues of the Nioo-Deities of the gate of the temple Todai-Ji (Toodai-Ji 東大寺).
These huge statues are placed with a lot of empty space above them. The sunrays reaching the gate will be reflected on the billowing robes and finally reach the face of each deity in an outburst of cosmic energy above their heads.

The statue A (阿形 agyoo) has his mouth open, the beginning of all things.
The statue UN (吽形 ungyoo) has his mouth shut, the end of all things.

These gates usually have two stories and in the lower part, behind wooden lattice, are the statues of the Deva Kings (kongooza 金剛座), facing toward the onlooker as he enters the temple compound and showing their angry face at all enemies of Buddhism.
There are very few temples where these Deva Kings face each other. In this case, then you stand inside the gate, they seem both to look at YOU rather sternly, asking to leave your negativity and bad thoughts all outside and cleanse your mind before going inside.

Sometimes people chew a piece of red or white rice paper, press it on a part of their body that hurts and then throw it at the deity in the gate. If the paper sticks to the right place of the deity, the person will be healed.

If children manage to climb through between the legs of these statues, they will grow up healthy and fast runners.

These deities have also been venerated for their speed, especially with the running post curriers of the Edo period (hayabikyaku 早飛脚（はやびきゃく）). Even now we find many straw sandals hanging at these gates.Temple Saikoku-Ji : My Daruma Haiku

Most of the old statues are rather damaged, since they have to withstand the weather of all seasons. Hence the one haiku by Kikaku.
Their strong physical features might go back as far as the Greek deity Hercules.

The clay statue of Mighty Diamond Deity, Shitsu Kongo Yasha (see stamp below) from 733 at the Sangatsu-Do Hall of the Todai-Ji temple compound is most famous. It has been kept a "secret statue" (hibutsu) inside and is therefore well preserved to our day.

Nioo mata kuguri 仁王尊股くぐり
During the three days of the New Year celebrations and at the big temple festivals in Spring and Autumn it is possible to crawl under the legs of the Nio statues, which is another special ritual not seen in Japan. It will ward off evil influence and kept you healthy, especially in times of smallpox in the Edo period.
It used to be a custom of local people, mostly bringing their sick children.
But later it spread and even Daimyo lords came to perform this ritual.

Ishite-ji (石手寺) is a Shingon temple in Matsuyama, Ehime Prefecture, Japan. It is Temple 51 on the Shikoku 88 temple pilgrimage. Seven of its structures have been designated National Treasures or Important Cultural Properties.- - - More in the WIKIPEDIA !

What's unique about this Fudo is that it can only be found in the dark cave behind Ishite-ji in Matsuyama. There are many different statues, so you have to use a flashlight to obtain clarity.- shared by Bradford on facabook

The Nio from India (唐天竺 Kara Tenjiku, China and India ) wanted to compare this powers with Shomen Kongo and went over to China for a match. But he could not win and had to flee further, until he finally came to Japan. Kongo came after him and Nio had to hide somewhere in Japan, so he made it to a temple and stayed at the gate. . Shōmen Kongō 青面金剛 Shomen Kongo .